1. Field of the Invention
The present invention relates to an imaging optical system, image forming apparatus having the same, and a method therefor, which are adapted for use in a printer, a copier, a facsimile machine, etc. More particularly, the present invention relates to an imaging optical system and image forming apparatus having the same, in which image beams, such as a laser beam, are emitted on an imaging surface of a photosensitive material by using a plurality of combined reflection mirrors.
2. Description of the Related Art
Generally, an image forming apparatus which is used in a printer, a copier, a facsimile machine, etc., has a laser scanning unit (LSU) to form an electrostatic latent image on a photosensitive material, such as a photosensitive drum. The image is formed by irradiating beams according to an image signal. The apparatus further has an image forming part to develop the electrostatic latent image formed on the photosensitive material into a toner image through a developer, such as colored toner, and transfer the image onto a sheet of printing paper to form a desired image.
To form the electrostatic latent image on the photosensitive material, the LSU has a light source having a plurality of luminous elements such as light emitting diodes (LED), laser diodes (LD), or vertical cavity surface emitting lasers (VCSEL), which are one or two-dimensionally arranged.
As an example of an LSU has a polygon mirror. The LSU changes beams emitted from the light source into parallel light beams through a collimating lens, leads the beams to the polygon mirror to deflect a reflective direction of the light beams, and then scans the beams as light spots conforming to a printing pattern on an imaging surface of the photosensitive material through an fθ lens to form the electrostatic latent image thereon.
However, the LSU employing the polygon mirror has a problem in that mechanical vibration and noise occur because of the mechanical rotation of a motor to rotate the polygon mirror during scanning. Specifically, in order to achieve a high scanning speed, the speed of revolution of the polygon mirror is increased, and the increase in noise is ever greater, and rotation accuracy and reliability of the motor are lowered because of an increase in a rotation load.
In order to overcome these problems, an LSU using an imaging optical system as a magnification optical system to image beams by using a plurality of combined lenses has been proposed. This LSU does not require a polygon mirror, and is shown in FIG. 1.
FIG. 1 illustrates an image forming apparatus 10 having a semiconductor laser array 1 in which a plurality of laser elements 1a are one or two-dimensionally arranged, and an imaging optical system 4 as a magnification optical system to magnify a plurality of laser beams emitted from the laser array 1 at a given angle and position . The apparatus 10 also includes a photosensitive drum 3 having an imaging surface to image the laser beams outputted from the imaging optical system 4 thereon, a drum driving circuit 5 to drive the photosensitive drum 3 at a given rotation speed and to output a timing signal synchronously with the rotation of the photosensitive drum 3, a signal processing circuit 7 to read an image signal from an image memory 6 and process the image signal to output a printing signal conforming to a printing pattern to a laser driving circuit 8, and a control circuit 9 to output a control signal to the laser driving circuit 8 synchronously with the timing signal from the drum driving circuit 5.
The imaging optical system 4 includes a first lens group 4a disposed at a photosensitive drum side of the imaging optical system 4 to image incident laser beams 2 traveling vertically from the laser elements 1a on the imaging surface of the photosensitive drum 3. The imaging optical system 4 further includes a second lens group 4b having a stop 4e and a composite focus 4d adjacent to the first lens group 4a toward the laser array 1, and a third lens group 4c disposed between the second lens group 4b and the laser array 1.
The second and third lens groups 4b, 4c form a telecentric optical system, so that the curvature of the laser beams 2 incident on the third lens group 4c and an aperture of the third lens group 4c can be reduced. Also, principal light beams 2a of the laser beams 2 from the laser array 1 are bent at two stages by the second lens group 4b and the third lens group 4c. 
During operation of the apparatus 10, according to a printing command from a computer or central processing unit (CPU) (not shown), the signal processing circuit 7 reads an image signal from the image memory 6 and processes the signal to output a printing signal conforming to a printing pattern to the laser driving circuit 8.
At this time, the drum driving circuit 5 controls the photosensitive drum 3 to rotate at a given speed of rotation and simultaneously outputs a timing signal synchronous with the rotation of the photosensitive drum 3 to the control circuit 9. As the drum driving circuit 5 outputs the timing signal, the control circuit 9 outputs a control signal to the laser driving circuit 8 synchronously with the timing signal.
In response to the control signal from the control circuit 9 and the printing signal from the signal processing circuit 7, the laser driving circuit 8 drives the laser array 1. At this time, the laser array 1 emits the plurality of laser beams 2 in parallel with the optical axis (direction Z). The laser beams 2 first enter the imaging optical system 4, and the laser beams 2 incident upon the imaging optical system 4 are bent by light gathering action of the third and second lens groups 4c, 4b. Then, the laser beams 2 are gathered at the composite focus 4d positioned at an imaging surface side of the second lens group 4b, and are scanned in a main scanning direction as a plurality of light spots on the imaging surface of the photosensitive drum 3. At this time, since the photosensitive drum 3 is driven to be rotated in a sub-scanning direction by a driving motor (not shown), an electrostatic latent image conforming to the image signal is formed on the photosensitive drum 3. This is the result of the movement of the light spots in the main scanning direction and the movement of the photosensitive drum 3 in the sub-scanning direction.
However, in the above-described image forming apparatus 10, to reduce an optical aberration, distances D1′, D2′ between the laser array 1 and the second and third lens groups 4b, 4c are set beyond certain values, thereby resulting in the total length of the optical system being increased. Also, as the distances D1′, D2′ between the laser array 1 and the second and third lens groups 4b, 4c are increased, an area of the laser beams 2 incident on the imaging optical system 4 is enlarged and as a result, an effective aperture of the lenses forming the first and second lens groups 4a, 4b is increased.
Thus, in a case in which the total length of the optical system and size of the lenses are increased, the total size of the image forming apparatus is enlarged, and at the same time, a weight thereof is increased. Also, when the lenses have large apertures, designing surface shapes of the lenses into aspherical surfaces becomes impossible, and manufacturing processes such as lens design, mold production, injection molding, etc. become difficult, thereby resulting in increased manufacturing time and cost.
Also, in the conventional image forming apparatus 10, at least three lenses are required in the telecentric optical system in order to realize a resolution of more than 600 dpi, This complicates the design and also increases manufacturing costs.
Also, since the conventional image forming apparatus 10 has refracting surfaces of the lenses which are disposed with rotational symmetry with respect to the optical axis to thereby reduce the asymmetrical aberration, magnification powers in the main and sub-scanning directions are equal. Thus, if the magnification powers in the main and sub-scanning directions should differ, additional structures are required.
Another problem of the conventional imaging optical system 10 is that temperature changes cause the wavelength of the laser beams emitted from the laser array 1 to change, and thus, the refractive index of the lens material is changed, and image quality deteriorates.